The present invention pertains to an electrochemical gas sensor with a plurality of electrodes, with an electrolyte and with a gas-permeable membrane.
Such a gas sensor is described in DE 196 39 311 C2, in which the measuring electrode consists of a corrosion-resistant material from the platinum group or of gold.
Highly sensitive and reliable gas sensors are needed for the detection and the measurement of the concentration especially of toxic gases, electrochemical gas sensors being particularly suitable for many gases and because of their robustness and long service life.
Such electrochemical gas sensors must meet in practice a number of requirements, e.g.:
the basic current should be low;
the basic current should be affected by changes in the moisture content in the air and the air temperature as little as possible;
the measured signal should have the highest possible long-term stability;
the gas sensor should have the lowest possible cross sensitivity for interfering gases that occur together with the gas to be measured.
In particular, the requirement for the lowest possible cross sensitivity can be met only insufficiently in many cases for a gas sensor and for the corresponding measuring electrode while the other quality criteria are met.
The object of the present invention is therefore to propose an improved gas sensor that measures more selectively with reduced cross sensitivity with respect to interfering gases but has at the same time a short response time and high sensitivity for the gas to be measured, without its service life being reduced.
According to the invention, an electrochemical gas sensor is provided with a plurality of electrodes, with an electrolyte and with a gas-permeable membrane. At least the measuring electrode is a thin layer consisting essentially of diamond-like carbon (DLC, diamond-like carbon) on the gas-permeable membrane.
The thickness of the thin layer of diamond-like carbon may be 50 nm to 1,000 nm. The thin layer of diamond-like carbon may be produced by a coating process at room temperature, preferably by a sputtering process in a radio frequency magnetron sputtering unit. An adhesion promoter, especially a silicon-containing adhesion promoter, may be applied between the thin layer of diamond-like carbon and the gas-permeable membrane. The electrolyte may contain a mediator, which selectively engages in a rapid reaction with the gas to be measured, the reaction yielding a product that reacts in a rapid, especially diffusion-controlled reaction at the measuring electrode and whose concentration is an indicator of the concentration of the gas to be detected. The mediator may be LiBr or Ag2SO4. The gas-permeable membrane may be a perfluorinated polymer, especially PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy polymer) or FEP (copolymer of hexafluoropropylene and tetrafluoroethylene) or of one of the materials Cyclopore(copyright), Isopore and Nuclepore(copyright).
According to another aspect of the invention a process for using an electrochemical gas sensor is provided for detecting F2, Cl2, ClO2, Br2, I2, NO2, H2O2, O3, PH3, and AsH3.
It was surprisingly observed in experiments that the properties of the gas sensor are sometimes greatly improved if the measuring electrode of an electrochemical gas sensor is designed in the form of a thin layer applied to the gas-permeable membrane preferably by means of a sputtering process in a magnetron from diamond-like carbon: Thus, the long-term stability is further improved, the influence of changes in the moisture content in the air and the air temperature on the basic current is reduced, and, in particular, the cross sensitivity to interfering gases is drastically reduced.
The essence of the present invention is that diamond-like carbon (DLC) is applied in a very thin layer to a gas-permeable membrane according to prior-art techniques. This arrangement is used in the gas sensor as a measuring electrode with a diffusion membrane arranged in front of it. The diamond-like carbon layer may be produced with an RF (radio frequency) magnetron sputtering process, but also by means of other coating methods. By selecting the process judiciously, it is possible to use both undoped and doped layers to improve the electric conductivity.
Even though diamond-like carbon layers (DLC, diamond-like carbon) have been used as an electrode material for some years now, as is apparent from the article xe2x80x9cCarbon-Based Materials for Microdevices,xe2x80x9d Microsensor Technology, mst news, 21/97, pp. 14 through 16, Andrxc3xa9 Perret et al., these layers were applied at high temperatures to gas-tight substrates, e.g., silicon, and were therefore unsuitable for use in gas sensors. It has been known from EP 0 647 318 B1 that a porous membrane can be coated with this material, but the use of the material layer according to the present invention as an electrode and especially as a measuring electrode is not embodied in this prior-art arrangement.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which preferred embodiments of the invention are illustrated.